The present invention relates to a method of and a device for driving a thermal head in a printer, and more particularly to a method of and a device for driving a thermal head in a printer, which thermal head comprises a linear array of heating elements arranged in a main scanning direction, by supplying the thermal head with a number of pulse signals depending on the gradations or tones of an image signal produced from an image signal recording medium such as a video floppy disk, thereby to record a two-dimensional image on a heat-sensitive body such as a heat-sensitive film, for example, which is being fed in an auxiliary scanning direction across and against the thermal head, the pulse durations of the pulse signals supplied to the thermal head being variable depending on changes in the temperature of the thermal head in order to reproduce the image in accurate rate gradiation or tones.
Various modern medical imaging diagnostic systems such as ultrasonic imaging systems, X-ray CT systems, and DF systems, in addition to the conventional X-ray imaging systems, have recently been used widely in the medical field. In such new medical imaging diagnostic systems, an ultrasonic or X-ray radiation is applied to a region of the body of a patient, and changes in the ultrasonic or X-ray radiation that has passed through the patient's body are detected to produce an image of the patient's body, which is typically displayed as a visible image on a CRT monitor. The doctor can then diagnose the region of the patient's body based on the displayed image. If necessary, other regions of the patient's body can easily be observed through the imaging system during the diagnostic process. The quick and easy imaging capability allows the diagnostic process to be performed accurately and quickly.
It is frequently necessary that the display data of the body regions which are displayed on the monitor be subsequently sent to another hospital or displayed so that any chronological changes in the imaged body region can be observed by the doctor and the patient together. To meet these requirements, the images displayed on the CRT monitor ar permanently recorded on recording mediums as hard copies. Usually, the displayed images are recorded by various printers. One of such various printers is a thermal printer which employs a light-fixable heat-sensitive film having a heat-sensitive material layer that can develop a color upon exposure to heat and can be fixed when irradiated with ultraviolet radiation.
The thermal printer accommodates a roll of light-fixable heat-sensitive film. As shown in FIG. 9 of the accompanying drawings, such light-fixable heat-sensitive film F is sent to a printing mechanism 2 which has a thermal head 6 comprising an array of as many heating elements 4 as the number of pixels to be formed, the array of heating elements 4 extending along a main scanning direction indicated by the arrow A. The light-fixable heat-sensitive film F is sandwiched between the thermal head 6 and a platen roller 8, which is rotated about its own axis in the direction indicated by the arrow B to feed the light-fixable heat-sensitive film F in an auxiliary scanning direction indicated by the arrow C. At the same time, the heating elements 4 are selectively supplied with a pulse signal based on an image signal transmitted from a medical imaging diagnostic system, thereby recording a two-dimensional image on the light-fixable heat-sensitive film F. Then, the light-fixable heat-sensitive film F is delivered to a fixing device (not shown) in which an ultraviolet lamp (not shown) is energized to apply ultraviolet radiation to the film F to fix the recorded image. The light-fixable heat-sensitive film F with one frame of image information recorded thereon is cut off to a predetermined length by a cutter (not shown), and the cut film F is fed into a discharge tray (not shown).
The light-fixable heat-sensitive film F thus cut off and carrying the reproduced image is schematically shown in FIG. 10a. The film F includes a hatched area which carries the reproduced image frame, the reproduced image being recorded in a printing area L.sub.0 in the auxiliary scanning direction indicated by the arrow C, i.e., in the direction in which the light-fixable heat-sensitive film F is fed.
When a pulse signal whose represented image density is a constant image density D.sub.0 (see FIG. 10b), i.e., a pulse signal representing a constant heat energy level, is continuously supplied to the heating elements 4 of the thermal head 6 with respect to all main scanning directions which cover the printing area L.sub.0, the image density D is gradually increased as indicated at D.sub.1 with respect to the constant density D.sub.0, and as a result the reproduced image has varying image densities. The inventor has found that such a phenomenon is caused by the heat storage effect of the heating elements 4 of the thermal head 6.